From the time of man's first cultivation of crops he has been at the mercy of the weather. Myriad schemes and devices have been employed to attempt to control the manner in which moisture and temperature affect cultivated land. A particular nemesis of cultivated land is the cold air which occurs naturally as the sun goes down. Particularly susceptible to the cool evening air are crops planted in the generally fertile valleys, since the warm air tends to rise and the cooler air tends to settle into the valleys. Furthermore, when it becomes so cool as to permit frost to form the crops are almost always destroyed. Over the years there have been various types of equipment used to prevent such crop damage, with the most successful being the simple combustion heater used to warm the surrounding air. Of course there are natural phenomena which also provide crop protection. Examples of these are fog, clouds, and atmospheric conditions wherein the dew point temperature is relatively high.
At present the most popular frost prevention system is to employ a series of combustion devices located in the cultivated area to heat the surrounding air. These are generally oil burning and produce relatively large amounts of hot air and hot exhaust products which act to prevent frost damage. However, a major problem existing with these combustion heaters relates to the fact that the hot air tends to rise and the cooler air tends to settle over the cultivated area. In other words, the hot air which is intended to keep the cooler air away from the crops tends to become more bouyant and to rise beyond the area intended for protection. This counter productive effect has been accepted as an inherent drawback in frost prevention systems, and most developmental work done in this area involves improved heater designs or automated systems of heaters.